1. Field of the Invention
This invention relates to a non-linear control element for a flat electrooptical display screen, especially for a liquid-crystal display screen, and also to a method o fabrication of said control element.
The present invention finds an application in the general field of large-area thin-film electronics and is more specifically applicable to integrated control of each elementary point of a liquid-crystal screen.
2. Description of the Prior Art
It is already known that liquid-crystal display screens usually have a large number of image points or elements of square or rectangular shape. These image elements can be addressed individually. The definition of the screen is a function of the number of points which are capable of receiving an item of information. Control of each point is performed by application of an electric field. For the purpose of visualizing video information, it has been proposed to provide matrix-type displays in which each image element is defined by the intersection of two orthogonal arrays of leads designated as rows and columns.
Addressing of an image element by means of control voltages applied to the row and to the column which relate to said element does not need to be maintained in the event of adoption of a time-multiplexing technique which permits refreshment of the state of the screen by recurrence. This technique is based on a persistence effect which may be either physiological or available within the screen element. In the case of liquid-crystal display devices, an image element can be assimilated with a capacitor in which the time constant is sufficient to maintain the charge between two successive transient addressing operations.
The design concept of matrix control by means of row leads and column leads in order to obtain high screen complexity (large number of rows) entails the need to mount in series with the image element a non-linear resistor which is practically insulating at values below a threshold voltage but becomes increasingly conductive above said threshold voltage.
A non-linear element of this type can be of varistor material as described in French patent application No. 81 16217 filed on Aug. 25th 1981 in the name of the present Applicant and published under No. 2,512,240.
In the field of display screens, current technical requirements are primarily centered on the achievement of higher image definition. In the case of screens of the matrix display type, it is accordingly found necessary to design devices comprising a large number of addressing rows or columns which can amount to as many as 512 or even 1024. This entails a corresponding increase in the number of switching elements and therefore of varistors. For the purposes of large-scale production, it is necessary in particular to obtain good reproducibility and high stability of these components. Moreover, the electrical capacitance of the component must be matched with that of the associated cell, also with good reproducibility. In point of fact, however, these requirements cannot be fully satisfied by the materials commonly employed, such as agglomerates of zinc oxide powder containing particles of bismuth oxide and manganese oxide or similar material. The reproducibility and stability of varistors depend among other things on the grain size and on the techniques of passivation of the grain boundaries employed at the time of fabrication. The stray capacitance of the varistor which is also related to the grain boundaries cannot readily be controlled.
Other switching elements can be employed. As a general rule, however, liquid-crystal display screens usually exhibit defective uniformity of contrast according to the image elements considered. Such defects are due to dispersion of the characteristics of the switching elements, which may be substantial and is difficult to eliminate on large areas. Although to a lesser extent, such defects may also arise from the thickness of the liquid crystal layer and from its bonding layer.
In order to overcome these disadvantages, devices are known in which the non-linear elements are thin-film transistors which mainly have a base of amorphous silicon or polycrystalline silicon. However, a certain number of difficulties are encountered in this type of technology and therefore have to be overcome if high-quality addressing is to be achieved. The solutions to be found must accordingly take into account the following considerations:
(1) better control of characteristics which depend on the properties of two layers (silicon and insulator) and of their interface;
(2) a self-alignment technology is necessary in order to achieve better reproducibility over a large area.
Other solutions contemplate the use of non-linear elements consisting of dipole elements such as the structure having a base of two diodes mounted in series and in opposition.
These diodes are semiconductor diodes which all have the same operating point in the current-voltage characteristic. Devices of this type are described in French patent Application No. 83 14542 filed by the present Applicant on Sept. 13th 1983 and are designed in particular in the form of Schottky diodes.
However, the practical application of this solution usually entails the need for at least four masking levels.
The present invention relates to a non-linear control element and to a method of fabrication of said control element in which the number of masking operations can be reduced to two and in which the fabrication of masks does not call for a high standard of accuracy. Furthermore, the control elements obtained in accordance with the invention occupy only a very small surface area and are therefore less visible than elements of known types in the present state of the art.